Crystalline polyamide resins as typified by nylon 6 and nylon 66 are widely employed as an engineering plastic due to their excellent mechanical properties and easiness of melt molding; however, a deficiency in heat resistance and a defective stability of dimension by absorbing water are pointed out. On the other hand, a polyphenylene ether resin excels in a heat resistance and a dimensional stability; however, the presence of problems is also pointed out, for example, a viscosity as molten is high and a molding processability and a chemical resistance are extremely inferior. For this reason, a technology of blending a polyamide resin and a polyphenylene ether resin has previously proposed (Patent document 1; JP-A-562-270654), and a study has been made on the creation of a new material combining the excellent properties that respective resins intrinsically have.
In order further to take advantage of an excellent molding processability, mechanical property, chemical resistance and dimensional stability which a polyamide-polyphenylene ether resin composition has, a study has also been made on an addition of an agent imparting electroconductivity for an application with a large size requiring an electrostatic coating step such as an automotive exterior part and the like (Patent document 2: JP-A-H02-201811).
Since by employing a polyamide resin produced by employing oxalic acid as a dicarboxylic acid component, the resin has a higher melting point and a lower percentage of water absorption in comparison with other polyamides with the same concentration of a amino group (Patent document 3: JPA-2006-57033), the resin is expected to be utilized in the field where it is difficult to use the conventional polyamides having defects in a heat resistance and a dimensional stability upon absorbing water.
It is well known that an electroconductive filler is kneaded and dispersed into an electrically insulative resin in order to impart electroconductivity for antistatic and other purposes, which in turn, an electroconductive resin is made. As an electroconductive filler to be kneaded into a resin, an ionic electroconductive organic surfactant, a metal fiber and powder, an electroconductive metal oxide powder, a carbon black, a carbon fiber, a graphite powder and the like are generally utilized. By molding and processing an electroconductive resin composition wherein this filler has been molten, kneaded and dispersed into a resin, a molded article having a volume resistance value of 10−1 to 1012Ω·cm can be obtained.
In terms of an electroconductive filler, a relatively small amount of blending can also impart an electroconductivity to a resin by using a material with a high aspect ratio (length/outside diameter) in a flake form, a whisker form or a fibrous form. This is because an electroconductive filler with the higher aspect ratio forms the more effective linkage between fillers in the same amount of blending, which allows for obtaining an electroconductivity in the smaller amount.
However, a metallic filler is inferior in a corrosion resistance and a chemical resistance. An inorganic electroconductive filler requires a large amount of blending more than 50% by mass relative to the total mass of a composition because it is generally granular. Thus, its resin properties degrade and molding becomes difficult. Blending with a carbon black in 15% by mass or less allows for a high electroconductivity because the Ketjen Black and an acetylene black are available, which form an electroconductive circuit with a chain-like structure. However, these are difficult to the control of dispersion into a resin and a distinct formulation and mixing technologies are required to obtain a stable electroconductivity. Even if a sufficient electroconductivity is obtained, not only a processability extremely degrades but also the physical properties of an electroconductive resin composition such as a tensile strength, a flexural strength and an impact resistance strength extremely degrade in comparison with the original physical properties of a resin free from an electroconductive filler.
When the carbon fibers with different fiber diameters are blended in the same amount of mass, the fiber with the smaller fiber diameter is more excellent in imparting an electroconductivity due to the more facile formation of an electroconductive circuit network among fibers. A hollow extra-fine carbon fiber, the so-called carbon nanotube has been recently disclosed, which has a fiber diameter smaller in two to three digits than that of conventional carbon fibers, and its blending into various resins, rubbers and the like has been also proposed as an electroconductive filler (Patent document 4: JP-A-H01-131251, Patent document 5: JP-A-H03-74465, Patent document 6: JP-A-H02-235945), which is regarded as an effective electroconductive filler solving the defects of the conventional electroconductive fillers.
These conventional ultrafine carbon fibers collectively called as carbon nanofiber or carbon nanotube can be generally categorized into the following three nanostructured carbon materials based on their shapes, configurations and structures:
(1) Multilayer carbon nanotube (multilayer concentric cylindrical graphite layer)(non-fishbone type);
Japanese publication of examined application Nos. H03-64606 and H03-77288
Japanese Laid-Open publication No. 2004-299986
(2) Cup stack type carbon nanotube (fishbone type);
U.S. Pat. No. 4,855,091
M. Endo, Y. A. Kim etc.: Appl. Phys. Lett., vol 80 (2002) 1267 et seq.
Japanese Laid-Open publication No. 2003-073928
Japanese Laid-Open publication No. 2004-360099
(3) Platelet type carbon nanofiber (card type)
H. Murayama, T. maeda: Nature, vol 345 [No. 28] (1990) 791 to 793
Japanese Laid-Open publication No. 2004-300631.
In a (1) multilayer carbon nanotube, conductivity in a longitudinal direction of the carbon nanotube is high because electron flow in a graphite network plane (C-axis) direction contributes to conductivity in a longitudinal direction. On the other hand, for inter-carbon-nanotube conductivity, electron flow is perpendicular to a graphite network plane (C-axis) direction and is generated by direct contact between fibers, but it is believed that within a resin, since inter-fiber contact is not so contributive, electron flow by electrons emitted from the surface layer of a conductive filler plays more important role than electron flow in fibers. Ease of electron emission involves conductivity performance of a filler. It is supposed that in a carbon nanotube, a graphite network plane is cylindrically closed and jumping effect (tunnel effect hypothesis) by π-electron emission little occurs.
In an ultrafine carbon fiber having a (2) fishbone or (3) card type structure, an open end of a graphite network plane is exposed in a side peripheral surface, so that conductivity between adjacent fibers is improved in comparison with a carbon nanotube. However, since the fiber has a piling structure in which C-axis of a graphite network plane is inclined or orthogonal to a fiber axis, conductivity in a longitudinal fiber-axis direction in a single fiber is reduced, resulting in reduced conductivity as the whole composition.
The so-called carbon nanotubes described above have also difficulty in uniform dispersion into a resin, and they are far from satisfactory because there are problems such as unspinnability (broken thread), filter occlusion at the discharge part of a molding machine, deterioration in the mechanical strengths such as the impact resistance of a molded article and its surface appearance due to the residue of the undispersed portion of carbon nanotubes as an aggregate in a resin. For this reason, blending and mixing the especial compositions and the particular surface modification treatments are needed, for example, surface modification treatment of carbon nanotube (Patent document 7: JP-A-2004-323738), and the restriction on the kind, composition and the like of resins, thus, further improvements are demanded.